For decades, geologists have debated the origins of an unmistakable curve in the Appalachian Mountains. Known to experts as the “Pennsylvania salient,” the bend begins in southern New York and northeastern Pennsylvania and extends across Pennsylvania to the border of Maryland. In this area, the ridges of the Appalachians turn from a roughly north-south orientation to an east-west orientation and then north-south again.

A recent study, authored by geologists from the University of Rochester and College of New Jersey, focuses on a block of dense, mafic volcanic rock beneath the bend. The scientists argued that this underground mass forced the mountain chain to shift as it formed hundreds of millions of years ago. As the North American and African plates collided, the North American plate began to fold and thrust upward. However, the mass of volcanic rock became a barrier and forced the mountains to push up around it.

Geologists have known about this mass for some time, but after analyzing new seismic and gravitational data, they concluded that the expanse of volcanic rocks was about 450 kilometers (286 miles) by 100 kilometers (124 miles). “What we didn’t understand was the size of the structure or its implications for mountain-building processes,” said Cynthia Ebinger, one of the study authors.